Room-temperature bonding performance of wafers using thin Ti films was examined as a function of waiting time tw in vacuum between film deposition and bonding. The surface free energy at bonded interface γ decreased as tw increased. However, results indicated that a γ value greater than 6.1 J m−2 is obtainable even at tw of 1 h or more at vacuum chamber base pressures lower than 1 × 10–6 Pa. Bonding apparatuses of three types with different vacuum base pressures were used for this study, but γ values obtained using the equipment showed an almost quantitatively identical reduction as the calculated total amount of residual gas adsorption on Ti film surfaces, Ga, increased. This finding indicates that the interface contamination evaluated as Ga played a dominant role in determining the γ reduction. Structural analysis revealed that the formation of reactive Ti–O layers on film surfaces prevented the crystal lattice rearrangement between Ti film surfaces, thereby leading to γ reduction. A few Langmuir of H2O adsorption prevented the crystal lattice rearrangement to a considerable degree. Moreover, surface roughness reduction of Ti films was effective at maintaining high bonding performance for a long waiting time.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials